1. Field of the Invention
This invention relates generally to radio receivers and, more particularly, is directed to a radio receiver using frequency synthesizer tuning that can provide the operator with a fine tuning feel similar to that given by a variable-capacitor tuned radio receiver.
2. Description of the Background
In radio receivers employing a frequency synthesizer in the tuning circuitry, a local oscillation signal is formed by a voltage-controlled oscillator (VCO) in a phase-locked loop (PLL) configuration. When the frequency-dividing ratio N of the variable frequency-dividing circuit that is part of the phase-locked loop is varied, the local oscillation frequency varies in response and a change in the reception frequency occurs.
Accordingly, in the frequency synthesizer receiver channel selection can be performed by simple key operations that drive the tuned frequency up and down. For example, the reception frequency can be sequentially and continuously changed by simply depressing an up-channel key or a down-channel key. Nevertheless, although this kind of tuning is modern in technique when channel selection is accomplished by the aforenoted key operations it has been found to be troublesome and wearisome for the listener/operator.
On the other hand, in the more old-fashioned radio receiver utilizing a so-called variable capacitor to perform channel section, the listener finds it more enjoyable to tune the receiver by rotating a knob, while listening to sounds emanating from the loudspeaker of the receiver. It has been found that the listener feels better about tuning the reception frequency in the variable capacitor type receiver than when tuning the frequency synthesizer receiver.
A frequency synthesizer receiver has been proposed in which a dial, that is, a knob, is provided so that the frequency-dividing ratio N of the synthesizer is varied in response to rotation of the dial, thereby changing the reception frequency just like the variable-capacitor tuner.
It is also known that in a phase-locked loop (PLL), when the frequency-dividing ratio N is changed the loop requires a finite lock-up time to stabilize itself relative to the frequency-dividing ratio N thus changed. During this lock-up time, the reception frequency is not stabilized and, therefore, muting of the output sound must be effected.
In an example based on the broadcast standards of the United States of America, if the reception frequency is varied by one step, for example 100 kHz, by changing the frequency-dividing ratio N by 1 in an FM radio receiver, the lock-up time is as short as 2 milliseconds. Nevertheless, when the reception frequency is changed a large amount all at once, for example, from 87.5 MHz to 108 MHz, by changing the frequency-dividing ratio N from the minimum value to the maximum value, the lock-up time will increase to approximately 200 milliseconds.
Furthermore, even when the reception frequency is changed step by step, in increments of 100 kHz by changing the frequency-dividing ratio N by 1 each time, if the dial is continuously turned at a relatively high speed, then the frequency-dividing ratio N is considerably varied in a short period of time, so that the required lock-up time will be extended just like the case where the frequency-dividing ratio N is changed from its minimum value to its maximum value all at once. In other words, if the frequency-dividing ratio N is increased by a very large amount per unit time, the lock-up time of the phase-locked loop increases unacceptably.
Therefore, in a synthesizer receiver employing a dial, muting is effected during operation of the dial and the duration of the muting period is in a range of from 500 milliseconds to 1000 milliseconds.
More specifically, when the dial is rotated by one tuning increment amount corresponding to 100 kHz in an FM receiver, a device that detects the rotation of the dial will produce a pulse Pr, as shown in FIG. 1A. Then, at the leading edge of the pulse Pr the muting is turned ON from its OFF state, as shown in FIG. 1B. Further, when a preceding muting period Ta, for example, 10 milliseconds, elapses after the muting is turned ON, the frequency-dividing ratio N whose value is changed by 1 is set in the phase-locked loop.
When a succeeding muting period Tb, for example, 500 milliseconds, elapses after the new frequency-dividing ratio N is set in the phase locked loop, the muting is turned OFF.
On the other hand, when the dial is rotated more quickly, a number of pulses Pr proportional to the rotational amount of the dial will be generated with a pulse width and cycle corresponding to the rotational speed, as shown in FIG. 2A. Referring to FIG. 2B, the muting is turned ON at a timing point of the first pulse PR of the string of pulses and, as shown in FIG. 2C, a frequency-dividing ratio N whose value is changed by 1 at every leading edge of a pulse Pr is set in the phase-locked loop. Just as in the single increment example, when the period Tb elapses following the time Ta, in which the frequency dividing ratio is changed, the muting is turned OFF.
Accordingly, in this previously proposed system regardless of the rotational speed of the dial, the duration of the period Tb will be longer than that of the lock-up time of the phase-locked loop with respect to all pulses Pr, so that the muting is performed without fail during the lock-up time period of the phase-locked loop. Nevertheless, if the muting period Tb is determined as described above, when the fine tuning is performed by slowly rotating the dial in the forward or backward direction or when the reception frequency is varied by slowly rotating the dial, the duration of the muting period, that is, the period in which no sound is produced, will increase, so that the user finds it difficult and annoying to make fine tuning adjustments to the receiver.